In this work the oxidation states assumed by Ir in oxide systems used as heterogeneous catalysts for water oxidation are determined by means of in-situ X-ray Absorption Spectroscopy (XAS). Using a highly hydrated iridium oxide film allows to having the maximum number of Ir sites involved in the electrochemical processes occurring at the catalysts while water oxidation (oxygen evolution reaction, OER) occurs. X-ray Absorption Near Edge Structure (XANES) spectra clearly indicates the co-existence of Ir(III) and Ir(V) at electrode potentials where OER occurs. This represents a fundamental step both in the understanding of the water oxidation mechanism catalysed by heterogeneous Ir oxide systems and in the possible tailoring of electrocatalysts for OER.